The present invention relates to internal combustion engines, and more particularly to suppressing an electrical spark during the exhaust stroke of a four-stroke small internal combustion engine, for use in generators, lawnmowers and other lawn and garden equipment and the like.
A four-stroke reciprocating engine has four strokes of the piston and two revolutions of the crankshaft for each engine cycle. The first stroke, or the intake cycle, occurs as the piston moves downward, creating a partial vacuum in the cylinder. In the intake stroke, the intake valve opens, allowing an air and fuel mixture to enter the cylinder. The second stroke, or the compression stroke, compresses the air and fuel mixture as the piston moves upward. At the end of the compression stroke, the spark plug is fired to ignite the air and fuel mixture typically just before the piston reaches top dead center. The third stroke is the power stroke or expansion stroke. In the power stroke, the air and fuel mixture burns and expands, forcing the piston downward. The fourth stroke, or the exhaust stroke, forces burned gases out of the cylinder through the open exhaust valve as the piston moves upward.
Small 4-stroke, spark ignition internal combustion engines often use either an inductive-magneto or capacitor-discharge ignition system that generates a spark plug arc during each engine flywheel revolution. The spark plug arc generated near the end of an engine's compression stroke is used to generate engine power, while the spark plug arc generated near the end of the engine's exhaust stroke does not have any practical value.
When an engine has been running and then coasts to a stop while the engine's ignition switch is in the off position, fuel and fuel vapors may accumulate in the engine's exhaust system. When attempting to restart an engine with fuel and/or fuel vapors accumulated in the engine's exhaust system, the mixture of fuel and fuel vapors may be ignited by the spark plug arc that is generated during the engine's exhaust stroke. When this occurs, a loud popping sound may be generated within the engine's exhaust system.
FIGS. 1 and 2 illustrate a typical prior art small 4-stroke internal combustion engine with overhead valve (OHV) configuration. A pull rope 4 is used to rotate a manual starter 8, causing rotation of a crankshaft 12. An intake valve 16 controls the flow of fuel between a port 17 and a combustion chamber 20. Valve 16 is of the usual poppet type having a head 24 that is alternately seated and unseated on a seat 28. The valve 16 is operated by a valve operating mechanism 32 that moves the valve 16 between its closed position and its open position. Mechanism 32 includes a valve stem 36 connected to the valve head 24. Valve stem 36 is confined to axial movement in a valve guide 40 typically pressed into an engine cylinder head 42 on an OHV engine or in the engine housing on side valve engines. The exhaust valve (not shown) is operated by a valve mechanism that is similar to mechanism 32.
Valve operating mechanism 32 also includes a return spring 48, a cam follower 56 and a cam 60. Cam follower 56 may alternately engage and disengage both the cam 60 disposed on a cam shaft 64 and a compression release assembly (not shown). The cam 60 includes a lobe portion 68. U.S. Pat. No. 5,150,674 issued to Gracyalny and assigned to Briggs & Stratton Corporation, the assignee of the present invention, discloses such a compression release assembly. U.S. Pat. No. 5,150,674 is incorporated by reference herein. A push rod 84 is moved by the cam follower 56 axially toward a rocker arm 76, the rocker arm 76 having a first portion 77 and a second portion 78. As the push rod 84 moves the first portion 77, the rocker arm 76 pivots about a rocker fulcrum 80, allowing the second portion 78 to in turn move the valve stem 36.
Spring 48 extends axially about valve stem 36 and is retained in place by a spring retainer 49. The spring force of spring 48 biases valve 16 to its seated or closed position. The spring force of spring 48 is opposed by the axial movement of the cam follower 56 that moves the valve 16 to its unseated or open position.
A variety of means have been considered in attempting to prevent spark plug arcing from occurring during a small engine's exhaust stroke. Typically, such methodologies require sophisticated and expensive electronic circuitry to determine the current stroke of the engine in order to blank the appropriate spark plug arc. Due to the relative degree of sophistication and associated high costs, previous methodologies are not cost effective for use on a low-cost internal combustion engine.